Fourteen Transport for London workers have now died of coronavirus.
The death toll includes nine bus drivers, three Transport for London workers, an Underground employee and a worker for one of TfL's suppliers, Mayor of London Sadiq Khan told Sky News.
This news comes as a new system of boarding buses is being trialled in London to reduce contact with drivers.
TfL announced it will launch a pilot scheme which will see passengers using the middle door to board, amid growing concern over the safety of drivers.
The trial will be in place from this week on several routes operated by bus firm Abellio out of its depot in Walworth, south-east London, including two that serve hospitals.
Passengers using London buses normally board using the front door and leave using the middle door, to smooth the flow of people.
The switch in policy adds to other new safety measures such as discouraging the use of seats near the driver and covering the holes in their protective screen with clear film.
Bus use in London has fallen by around 85% due to the coronavirus pandemic.
TfL director of bus operations Claire Mann said: "London's hard-working transport workers are making a heroic effort at the frontline of the fight against this pandemic, and it is only right we consider everything we can to protect them.
"We've already delivered many other enhanced safety measures and by trialling middle door-only boarding on buses we can gain the information we need to see if we can further improve safety on London's buses.
"Most Londoners can do their bit to protect our bus colleagues and other critical workers by remembering: stay home, don't travel, save lives."
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